The present invention relates to a method and apparatus for three dimensional video sampling or scanning and, more particularly, but not exclusively to a method or apparatus for obtaining scanning data in real time.
There are many ways to scan the 3D shape of objects, and some of these methods have been in use for many years. One such method is to obtain shape information from shading, but this requires prior knowledge of how the object is lit. A method that has been in use for many years is shape from stereo, which involves using two cameras to photograph an object. A further method is to obtain the shape from photometric stereo. Photometric stereo uses successive images taken from the same camera position but under different lighting conditions.
A further technique is illustrated in appended FIGS. 1a) and 1b) and obtains shape from structured light. Briefly the technique involves constructing a surface model of an object based on projecting a sequence of well defined light patterns onto the object. For every pattern an image of the scene or object is taken. This image, together with the knowledge about the pattern and its relative position to the camera are used to calculate the coordinates of points belonging to the surface of the object.
There are several variants of the Shape from Structured Light technique. That illustrated in FIG. 1 involves projecting a plane onto an object 10 using laser light 12 (FIG. 1a). The image of such a scene is controlled to contain only the line 12 which represents the intersection of the object and the laser plane. Such an image is shown in FIG. 1b).
In order to reconstruct the entire object 10 the laser plane has to be projected onto different parts of the object, and this may be achieved by either moving the laser or moving the object. In one approach, multiple views of the object are obtained by rotating the object on the turntable. It is clear that the approach is not suitable for real time operation.
Another approach currently being used is known as shape from coded light. Referring now to FIG. 2, and the system involves projecting rapidly changing patterns from a projector 14 onto the object and then noting which patterns arrive at which pixels in a detecting camera 16. Pixels at which earlier projected patterns arrive can be assumed to be located deeper than pixels at which later projected patterns arrive. A processor unit 17 carries out the depth pattern decoding, allowing output 18 to display an image with 3D information.
An example of this approach is found in Song Zhang and Peisen Huang, High Resolution Real Time 3D Shape Resolution, New York State University, The paper describes a high-resolution, real-time 3D shape acquisition system based on structured light techniques. The system described uses a color pattern whose RGB channels are coded with either sinusoidal or trapezoidal fringe patterns. Again with reference to FIG. 2, when projected by a modified DLP projector, 14, with color filters removed, the color pattern results in three grayscale patterns projected sequentially at a frequency of 240 Hz. A high-speed black and white CCD camera 16 synchronized with the projector captures the three images, from which the 3D shape of the object is reconstructed. A color CCD camera (not shown) may also be used to capture images for texture mapping.
The maximum 3D shape acquisition speed is 120 Hz (532′500 pixels), which is high enough for capturing the 3D shapes of moving objects. Two coding methods, sinusoidal phase-shifting, and trapezoidal phase-shifting, were tested. The trapezoidal phase-shifting algorithm is reported to make real-time 3D reconstruction possible.
The above-described technique is experimental, however, laser scanners that can be found in some commercial products can be classified as part of the structured light technique.
In order to obtain real time 3D sensing, several new techniques have been recently developed. In addition to the trapezoidal phase shifting referred to above, the 3D structure of an object can be computed from the optical recorded deformation of a single known pattern. However, texture within the object may cause matching problems and significant inaccuracies.
Another interesting idea is that of 3DV-systems. There system involves flooding a target with rapid pulses of light. The pulses are reflected from the target in such a way that reflections arrive first from parts of the target closest to the camera. Reflections from more distant parts arrive later. The system is based on measuring the travel time of a pulse of light. The 3DV-systems products require very high precision and are thus very expensive. Furthermore they are sensitive to textures, albeit less so than with coded light.
There is thus a widely recognized need for, and it would be highly advantageous to have, a 3D scanning system devoid of the above limitations.